This invention relates to a disc cartridge for an MD (mini disc) or the like having a disc-like medium such as an optical disc or the like received therein, and more particularly to such a disc cartridge wherein an upper plate and a lower plate are connected to each other through a center frame interposedly arranged therebetween.
A record medium on which recording and reproducing of data are carried out using magnetism or light is typically received in a casing structure called a disc cartridge, resulting in being charged in a recording and reproducing apparatus provided with a device called a magnetic head or an optical pickup device while being kept received in the casing structure. The disc cartridge prevents dust or contaminant from adhering to the record medium, to thereby contribute to an improvement in operational reliability of the record medium.
For the purpose of carrying out recording and reproducing of data on the record medium in the recording and reproducing apparatus, it is required to keep positional relationship between the record medium and the recording and reproducing device constant to ensure reliability in the recording and reproducing operation. To this end, it is of course required to stably rotate the record medium. Thus, it is generally carried out to adopt a means for providing a surface of the record medium with satisfactory smoothness and flatness.
There are considered factors which deteriorate positional relationship between the record medium and the recording and reproducing device. The factors include vibration externally applied to the recording and reproducing device, vibration generated by a motor incorporated in the recording and reproducing apparatus, and the like. A failure in adoption of any suitable measure against such vibration leads to a failure in satisfactory recording and reproducing. In particular, although a vibration-proof measure is adopted in a recording and reproducing apparatus mounted on an automobile or the like which generates vibration of an increased magnitude, no vibration-proof measure is taken in the disc cartridge itself.
Resin increased in specific gravity has been used for providing a cartridge for a magnetic record medium such as a magnetic tape with increased functionality and quality such as increased sound reproducing characteristics, increased depth characteristics and the like.
More particularly, Japanese Patent Application Laid-Open Publication No. 325470/1993 proposes a magnetic record envelope formed of thermoplastic resin containing of barium sulfate in an amount of 1 to 30% by weight. Composite resin having barium sulfate added thereto in such an amount has specific gravity within a range of from 1.1 to 1.6. Also, Japanese Patent Application Laid-Open Publication No. 307859/1993 discloses a video tape cassette made of a composite resin material of 1.3 to 1.4 in specific gravity containing barium sulfate in an amount of 25 to 35% by weight. As a result of a study by the inventors, it was found that specific gravity within the ranges described above fails to permit the envelope and cassette to exhibit satisfactory vibration-proof characteristics.
Japanese Patent Application Laid-Open Publication No. 119084/1988 discloses a cassette half made of a polypropylene resin composition of 2 or more in specific gravity which contains an inorganic reinforcing material in an amount of 45% by volume of the composition. The organic reinforcing material contains zinc oxide in an mount of from 20 to 100% by weight. Also, Japanese Patent Application Laid-Open Publication No. 57182/1987 discloses a composite plastic material having at least one of calcium carbonate and barium sulfate incorporated therein in an amount of 45 to 65% by weight. The amount of such fillers to be added is said to be determined depending on moldability of the material by injection molding. As a result of a study thereon by the inventors, it was found that an increase in flowability or fluidity of the material or resin facilitates molding of an article complicated in structure due to formation of a number of fine ribs or the like such as a cartridge for a magnetic record medium while providing the article with increased dimensional accuracy and improved appearance and ensuring increased productivity of the article. Also, it was found that the above-described amount of addition of the fillers is excessive for a composite resin material for a magnetic record medium cartridge.
Also, Japanese Patent Application Laid-Open Publication No. 117768/1984 discloses a cassette made of a composite resin material having specific gravity of from 1.5 to 2.2. The publication teaches that zinc oxide, barium sulfate, lead sulfate or the like may be used as an additive or filler for the composite resin material and the amount of the additive or filler may be adjusted within a range of from 40 to 80% by weight. However, it fails to provide any example for illustrating the teaching.
Further, techniques for constructing a cartridge into a three-layer structure is disclosed in Japanese Patent Application Laid-Open Publication No. 113466/1982, Japanese Patent Application Laid-Open Publication No. 78478/1989 and Japanese Utility Model Publication No. 13668/1982. More specifically, Japanese Patent Application Laid-Open Publication No. 113466/1982 merely shows a disc cartridge in drawings thereof. It fails to teach a structure of the disc cartridge. Japanese Patent Application Laid-Open Publication No. 78478/1989 discloses a disc cartridge constructed of a center frame made of a material increased in rigidity and upper and lower plates made of resin. However, it fails to consider vibration-proofness. Japanese Utility Model Publication No. 13668/1982 discloses a tape cartridge constructed of a center frame made of aluminum by die casting and upper and lower plates made of resin. Unfortunately, formation of the center frame by die casting deteriorates productivity of the center frame and therefore the tape cartridge.
A disc cartridge is generally required to exhibit increased reliability in recording and reproducing of a signal. For this purpose, it is required to form the disc cartridge with highly increased configurational accuracy and dimensional accuracy. Also, the disc cartridge is required to exhibit such vibration-proofness as described above in order to stabilize positional relationship between a recording and reproducing device and a record medium. A disc cartridge is generally constructed of two members called an upper half and a lower half, which are formed by injection molding. More specifically, as shown in FIG. 1, a disc cartridge 100 includes an upper half 110 and a lower half 120 which includes ribs 130 for defining a space in which a record medium is received and ribs 140 for arranging an erasure preventing member in the lower half 120. In order to satisfactorily attain molding of the lower half 120 while ensuring configurational and dimensional accuracy thereof, it is required that resin used for the molding exhibits increased flowability or fluidity.
Injection molding generally takes place by injecting a molding resin material into a die. For injection molding of resin deteriorated in flowability, it is required to increase a temperature of the resin to increase flowability of the resin or increase a pressure of the resin, to thereby force the resin into the whole die. Unfortunately, a molded article thus formed is deteriorated in both dimensional accuracy and configurational accuracy. Thus, in order that an article such as a disc cartridge having a complicated and fine structure is molded with increased accuracy, it is highly required that the resin exhibits satisfactory flowability or fluidity. More particularly, in order to ensure operational reliability of a disc cartridge and improve productivity thereof, it is essentially important to use resin moldable as much as possible. Even a slight variation in flowability of the resin substantially affects appearance and dimensional characteristics of the disc cartridge, as well as productivity thereof.
As will be noted from the foregoing, there have been made a lot of proposals which attempt to ensure vibration proofness of a disc cartridge by selection of specific gravity of a molding material or resin and addition of a filler to the resin. However, the prior art substantially fails to fully consider a reduction in flowability or fluidity of the resin due to addition of the filler thereto or a deterioration in moldability of the resin, leading to a failure to provide a disc cartridge which is capable of meeting requirements on both vibration proofness and moldability.
Also, an increase in amount of the filler to be added to the molding resin causes the disc cartridge to be brittle, resulting in the disc cartridge being decreased in resistance to shock.
Now, the conventional disc cartridge of a three-piece structure disclosed in Japanese Patent Application Laid-Open Publication No. 78478/1989 described above will be described with reference to FIG. 2A. The disc cartridge indicated at reference numeral 200 includes an upper plate 230 and a lower plate 210 with a center frame 220 being interposedly arranged therebetween, resulting in being constructed into a three-piece structure, leading to an increase in rigidity. In the disc cartridge 200 thus constructed, a reduction in thickness thereof requires to reduce a thickness of the center frame 220. Unfortunately, a decrease in thickness of the center frame 200 deteriorates flowability of resin during molding of the disc cartridge.
The center frame 220 has a recess 221 of a substantially square-like shape defined therein for receiving a disc-like record medium 240 therein. However, formation of the square-like recess 221 in the center frame 220 causes corners of the center frame 220 to be decreased in thickness, leading to a deterioration in rigidity of the center frame 220. In order to avoid such a problem, a center frame designated at reference numeral 320 in FIG. 2B is provided, which is formed with a circular recess 321 for receiving a disc-like record medium 240 therein. The circular recess 321 permits corners of the square-like center frame 320 to be increased in thickness. Unfortunately, this causes a thickness of a central portion of the center frame to be reduced, to thereby still fail to ensure satisfactory rigidity of the center frame 320.
Also, the disc cartridge disclosed in Japanese Patent Application Laid-Open Publication No. 78478/1989 is so constructed that the upper and lower plates are secured to the center frame by means of screws. In addition, assembling of the disc cartridge is carried out by mounting the center plate on the lower plate to form a subassembly, incorporating the record medium, a plug, a shutter lock and the like in the subassembly, and mounting the upper plate on the subassembly.
Thus, in the disc cartridge disclosed, the upper and lower plates of a square-like configuration are screwed to the center frame of a square-like configuration. This results in the disc cartridge being substantially identical with a disc cartridge of a two-piece structure constructed of only an upper plate and a lower plate. Thus, in order to increase rigidity of the disc cartridge, it is required that the center frame is formed of resin having a reinforcing material added thereto. For the same purpose, it is also required that a region of the center frame in which the disc-like record medium is received is increased in thickness. Unfortunately, this causes an increase in whole thickness of the center frame, as well as generation of sink marks in a thin-wall region of the center frame, leading to a deterioration in appearance. It would be considered to form the center frame with lightening holes in order to avoid an increase in thickness of the center frame and generation of the sink marks. This causes a reduction in rigidity of the center frame although it improves appearance of the center frame. The center frame is subject to warpage depending on formation of the lightening holes, resulting in configurational accuracy of the center frame being deteriorated, leading to a failure in satisfactory assembling of the disc cartridge.